Signal processing systems such as communication receivers often must recover a desired signal which has been transmitted through a channel degraded by multipath. In order to compensate for the signal impairment introduced thereby, receivers can use signal processing techniques which estimate the channel conditions. However, this poses challenges for channels which are changing quickly, as is the case, for example, when the receiver is mobile and moving at high speed, i.e., due to high Doppler conditions.
Orthogonal frequency division multiplexing (OFDMA) systems, such as DVB-T often provide pilot tones for the purpose of making channel estimation easier. However, the sparseness of these tones renders it difficult to estimate the channel quickly and with efficient memory usage and calculations.
FIG. 1 is a high-level block diagram of an OFDM system which employs channel estimation, as known in the prior art. Data modulated by modulator 120 and pilot tones 122 are inserted in inverse Fourier transform block 102. Block 104 adds cyclic prefix to the output of inverse Fourier transform block 102 and supplies its output to Rayleigh channel 106. The pilot tones are interspersed periodically in the subchannels to enable channel estimation block for the Rayleigh channel 106. Signal sm received by Rayleigh channel 106 for transmission may be defined as shown below:
                                                        s              m                        ⁡                          (              t              )                                =                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                          a                                  k                  ,                  m                                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  k                  ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  f                  ⁢                                                                          ⁢                  t                                                                    ,                              mT            S                    ≤          t          <                                    (                              m                +                1                            )                        ⁢                          T              S                                                          (        1        )            
After the Rayleigh channel, the received signal is defined as below:rm=sm(t)h(t)  (2)where the channel response h(t) is defined as:
                              h          ⁡                      (                          t              ,              τ                        )                          =                              ∑            k                    ⁢                                                    γ                k                            ⁡                              (                t                )                                      ⁢                          δ              ⁡                              (                                  τ                  -                                      τ                    k                                                  )                                                                        (        3        )            
In the frequency domain, the received signal can be expressed asxl,m=Hl,mal,m+wl,m  (4)whereHl,m=Σγke−j2πfτk  (5)is the frequency response and wl,m represents the Additive White Gaussian Noise (AWGN). For the pilot subcarriers, the temporal channel can be obtained in accordance with the following expression:Ĥl,m=xl,ma*l,m=Hl,m+wl,ma*l,m  (6)
Block 108 removes the cyclic prefix from the received signal. Fast Fourier transform block 110 converts the time domain signal supplied by block 108 to a frequency domain signal. Block 112 extracts the pilot symbols from the signal supplied by block 110 to estimate the channel. In the above representation, the channel is assumed to be an ideal channel plus noise. To estimate the channel, the noise is suppressed and pilot interpolation is performed.
FIG. 2 shows how pilot channels are distributed among subchannels in the DVB-T/H system. DVB-T supports 2K and 8K OFDM subchannels, whereas DVB-H also supports a mode with 4K subchannels. With each successive symbol, the location of most of the pilot tones (alternatively referred to herein as pilots) changes, with the exception of the pilot tones at locations called continuous pilot locations. In this system, the pilots are inserted every 12 subcarriers in the frequency domain; this density of pilot tones is insufficient for noise suppression. One method for increasing the density of channel estimates is to use pilot tones located nearby in time and frequency to interpolate the values in between. This can be done using previous and future OFDM symbols to fill the pilots from 1/12 total subcarrier density to ⅓ total subcarrier density. However, conventional interpolation techniques are inefficient and require excessive amount of memory space to store data required to perform such interpolations. Conventional channel estimation techniques are not well suited for estimating the channel for sub-channels positioned near the edges of the channel.